An organic EL element constituting an organic EL device is a semiconductor element that converts electric energy into optical energy. In recent years, research using organic EL elements has been carried out in an accelerated manner and, by improvement in the organic materials and the like constituting the organic EL elements, outstanding reduction in the driving voltage of the elements is achieved, and also light-emission efficiency is enhanced.
An organic EL element has a construction such that an organic light-emitting unit layer is interposed between the positive electrode and the negative electrode for applying voltage. At least one of the positive electrode and the negative electrode is made of a transparent conductive material for emitting the light generated within the element to the outside. Typically, a transparent conductive layer is used as the positive electrode, and the light is emitted from the positive electrode side. As a material of the transparent conductive layer, for example, an extremely thin film of a metal such as Ag or Au, or a metal oxide such as indium tin oxide (ITO) or aluminum-doped zinc oxide (AZO) is used. These transparent conductive materials generally have a higher resistance as compared with materials constituting the metal electrode layer that does not require transparency, and an electric potential difference that varies with the distance from an electric current supplying terminal (typically, a positive electrode terminal) is generated within the film surface. For this reason, at parts that are distant from the electric current supplying terminal, the voltage applied to the organic light-emitting unit layer drops, leading to decrease in the brightness as compared with parts closer to the electric current supplying terminal. In particular, in an illumination device, it often happens that the whole surface of the element is configured to emit light at the same brightness, so that, when the element is configured to have a larger area, there will be variations in the brightness in the light-emitting region of the illumination device because of a decrease in the brightness in the region where the voltage drop has occurred.
As means for solving this problem, a method of providing an auxiliary electrode layer made of a metal having a good electric conductivity together in the transparent conductive layer is proposed (for example, Patent Document 1). However, with such a construction, there is a need to form an auxiliary electrode layer separately from the transparent conductive layer functioning as a positive electrode and the metal layer functioning as a negative electrode. Moreover, there is a need to provide an insulating layer for preventing conduction between the auxiliary electrode layer and the metal electrode layer. Also, there is a need to pattern the auxiliary electrode layer with use of a mask or the like, thereby raising a problem of making the production steps cumbersome.